Estimating the residual fatigue lifetimes of impact‐damaged composites using thermoelastic stress analysis

A new experimental method is presented for quantifying impact damage and estimating the remaining fatigue lifetime of impact damaged polymer matrix composite materials. The procedure is demonstrated using composites of glass fiber reinforced polyurethane produced by injection molding and structural reaction injection molding. Thermoelastic stress analysis (TSA) was used to quantify the stress concentration associated with impact‐damage in test samples of each composite. Following impact and TSA imaging, the samples were fatigued to failure over a range of stress amplitudes. The TSA‐derived stress concentration factors were used to determine a modified stress amplitude that collapsed the impact‐fatigue data onto a master stress‐life curve. This approach provides a quantitative measure of impact damage and a practical methodology for estimating the residual fatigue lifetime of impact; damaged composites.     

The effect of interfacial properties on damage evolution in model composites

This study explores the effect of interfacial properties on damage evolution and damage mechanisms in model glass fiber reinforced epoxy matrix composites. The composite properties were varied by changing the interfacial bond between the fiber and the matrix. Double‐edge‐notched specimens were tested in tension and evaluated using thermoelastic stress analysis (TSA) to observe damage initiation and evolution, and to identify the operative damage mechanism. Changing the interface properties was found to change the operative damage mechanism. This study conclusively demonstrates that the damage mechanism in composites can be controlled by judicious changes in the interface properties. POLYM. COMPOS. 26:241–246, 2005. © 2005 Society of Plastics Engineers.     

Ultrasonic Evaluation of Initiation and Development of Oxidation Damage in Ceramic-Matrix Composites

In this paper we report on the development of a method for ultrasonic nondestructive characterization of oxidation damage in ceramic-matrix composites. The method is based on ultrasonic measurement of elastic moduli of the composite, which are then used to determine the elastic moduli of the fiber-matrix interphase. Thus the interphasial damage may be estimated quantitatively. As a model system we used, to demonstrate applicability of the method, a unidirectional SiC-fiber-reinforced reaction-bonded silicon nitride matrix composite (SiC/RBSN). The composite samples were oxidized in flowing oxygen for 0.1, 1, 10, and 100 h at 600°, 900°, 1200°, and 1400°C. The ultrasonic phase velocity in the composite was measured at room temperature before and after oxidation; the data were then used to find the composite moduli, which quantify the induced damage. Significant changes in ultrasonic velocities and composite moduli were found as a result of oxidation. Fiber-matrix interphasial moduli were determined by multiphase micromechanical analysis. We found that oxidation of the carbon interphasial layer is the dominant mechanism in decreasing the elastic moduli of the composite. The critical exposure time for transition from the nondamaged to the damaged state at different oxidation temperatures has been determined.     

Composite machining damage quantification using thermoelastic stress analysis

A new experimental method is presented for quantifying machining damage in polymer matrix composites. The method consists of capturing infrared images of machined samples and using thermoelastic stress analysis to quantify damage from the machining event. A modified stress concentration factor is presented as an easily measured and useful damage parameter. Circular holes were drilled into the center of plate specimens fabricated from a commercially available glass fiber reinforced composite. A standard drill bit, brad point drill bit, and abrasive water jet machining were the three machine tools investigated. Infrared images were used to quantify the machining damage by assigning a thermoelastic stress analysis based stress concentration factor (mSCF) to each machined hole. The mSCF was then used to rank the damage inherent to each machining method. Optical and electron microscopy were utilized to identify the types of damage associated with the three machining methods. Finally, each sample was fatigued to failure to substantiate the IR results. The ranking of damage based upon the mSCF was in good agreement with the fatigue lifetime rankings: higher mSCF is associated with shorter fatigue lifetimes.     

Small-angle X-ray study of particulate reinforced composites

Small-angle X-ray scattering technique can be used to quantify the microvoids structure within a particulate reinforced composite. An expression for the correlation function of three-phase systems has been derived in terms of the correlation function of the individual phases. By using this expression and the scattered intensities from the damaged and the undamaged composites; it has been show that the volume fraction and the chord length of the microvoids can be obtained, provided no damage occurs to the reinforcement particles. In cases where the microvoids are preferentially oriented within the composites, an approximation scheme based on a linear transformation method has also been developed to measure the aspect ratio of the microvoids provided the volume fraction of these microvoids is much smaller than the other two phases.     

Effect of thermal residual stress on the tensile properties and damage process of C/SiC composites at high temperatures

Due to the mismatch of the thermal expansion coefficients between the matrix and yarns, thermal residual stress will appear in C/SiC composites. In this paper, a progressive damage model was used to predict the thermal-mechanical behavior of C/SiC composites and reveal the failure mechanism. Firstly, the properties of the composites under tensile load were tested at three different temperatures in vacuum. Then, the elastic-plastic progressive damage constitutive laws were used and implemented by a user-defined subroutine UMAT in ABAQUS. The thermal residual stress evolution in the cooling and heating processes was characterized. Finally, the stress-strain curves of the composites under tensile load at different temperatures were studied. The effects of thermal residual stress on the tensile properties and progressive damage process of C/SiC composites were revealed sequentially. This work can give design guidance for strengthening of C/SiC composites.     

Measurement of Fiber/Matrix Interfacial Shear Stress at Elevated Temperatures

An apparatus for measurement of the fiber/matrix interfacial shear stress at temperatures up to 1100° is described. Equipment was used to measure interfacial properties as a function of temperature in two ceramic-matrix composites and one metal-matrix composite. In the composites where the thermal expansion of the matrix was higher than that of the fiber, the interfacial shear stress decreased with temperature. The opposite trend was observed in a system with low matrix thermal expansion. The change of the interfacial shear stress with temperature of all the composites studied can be fully explained by considering the fiber/matrix expansion differences.     

Stress distribution in carbon fiber‐reinforced epoxy composites under the supercritical condition

The recycling of carbon fiber‐reinforced resin matrix composites is attracting more and more attentions, and many mechanical, chemical, and energy recycling methods have been proposed. Among those methods, supercritical solvent recycle technique has been developed recently. Hygrothermal treatment was applied first to the composites before the supercritical process. Numerical simulation method was used to carry out the calculation of the thermal stress, peel stress, and interfacial shear stress under both the hygrothermal and supercritical conditions, to identify the mechanism of structural damage of composites. It shows that the hygrothermal treatment reduced both the critical pressure and temperature, and accelerated the structural damage of composites under the supercritical condition. The temperature and pressure simultaneously impacted the mechanics performance of composites, while the temperature was the dominant factor to destroy the composites' structure. And, potential damage areas were located in the free end with two failure types mainly: interfacial debonding failure and stratified destruction. POLYM. COMPOS., 36:961–968, 2015. © 2014 Society of Plastics Engineers     

Thermal conductivities of composites in several types of dispersion systems

We measured thermal conductivities as well as electric conductivities of some composites in several types of dispersion systems. The dispersion state, that is, the ease in forming conductive chains in these composites, was estimated by the characteristic electric conductivities and compared with the thermal conductivities. Thus, it became clear that thermal conductivity of a composite was significantly affected by the dispersion state in the composite. Further, it was confirmed that the predictive model proposed in the previous report was adaptable to the thermal conductivity of the composites in several types of dispersion systems. It was made clear that the dispersion state of a composite affected the values C₁ and C₂ in the previous model.     

Influence of chemical and physical properties of the last generation of silicon carbide fibres on the mechanical behaviour of SiC/SiC composite

SiC/SiC composites reinforced with near stoichiometric SiC ceramic fibres (Hi-Nicalon S and SA3 Tyranno fibres) are attractive materials to be used in nuclear environment. Netherless, their mechanical properties must be improved and controlled. For example, SA3 Tyranno fibres (TSA3) -reinforced composites exhibit a brittle behaviour whereas composites reinforced by Hi-Nicalon S (HNS) fibres exhibit a conventional damage tolerant response. This difference is related to the nature of the fibre/matrix (F/M) coupling. The aim of this work was to identify the SiC fibres characteristics influencing the F/M coupling and consequently the mechanical properties of the composites. The experimental results point out that the TSA3 fibres exhibit a granular and rough surface leading to an increase of the residual stress and the interfacial shear stress in the SiC/SiC composites. Beside the roughness, the experimental results also point out that the surface chemistry of the SiC fibres significantly influence the F/M bonding.     

复合材料的仿真设计方法的构思

复合材料已经应用于很多工业领域，为扩大复合材料的需求，准确评价它的机械性能对于评价其安全性是非常重要的，但复合材料的断裂模式和机理复杂，作这个评价产不容易。我们以损伤力学为基础，提出了一种复合材料性能的评价方法，由于织物复合材料结构复杂，可以想像，我们不能轻易地评价损伤过程中复合材料的机械性能。因此，本文给出一种方法，通过模拟含有某些损伤的复合材料的机械特性。这些损伤包括纤维束内的横向断裂或纤维束之间的剥离。在拉伸载荷下纤维纺织复合材料的第一个断裂发生在横向纤维束内，可以确认计算结果同实验结果一致。此外，我们提出了一种新的试验方法，可以得到各向异笥材料的偏轴强度。它是通过旋转支撑销的产生简单弯曲，旋转销可以在试件上产生均匀的应力状态，因而得到精确的强度。另一方面，缠绕已经广泛应用于纤维增强复合材料的生产工艺中。在缠绕中，纤维取向量人上很重要的设计参数。但是，纤维取向与芯模形状有关，预测纤维取向并不容易。因此，用于片状缠绕结构的仿真生产系统得到发展。一般来说，这种系统对于普通技术人员必须容易掌握的。所以，创立了图像用用户界面（GUI），使设计更加方便，结果这个系统对用户非常。本文讨论了算法和模型化方法，并且展望了这种系统的可用性。     

Localized functionally modified glass fibers with carbon nanotube networks for crack sensing in composites using time domain reflectometry

An electric time domain reflectometry (TDR) based sensing approach with an external parallel plate transmission line has been developed to evaluate high-frequency electromagnetic changes in composites due to applied load and internal damage. A model system of cross-ply glass fiber/vinyl ester composites with and without the selective integration of localized carbon nanotube (CNT) networks was studied where microcracking and delamination are introduced during tensile loading. A sizing technique has been used for localized functional modification using CNTs. The TDR sensing approach has been correlated with strain and acoustic emission (AE) measurements as well as micrographs of edge replicas capturing the damage state. Both the nanotube modified and baseline composites have similar mechanical properties and damage progression which is reflected in similar stress–strain plots, AE measurements and edge replica studies. However, the CNT introduced composites have enhanced strain and damage dependent TDR response. Hence, through localized functional modification of the composite electromagnetic properties using CNTs and the electromagnetic–mechanical property coupling of CNTs, it is possible to (1) increase TDR sensitivity to strain and (2) sense development of micro-scale cracks. This approach offers potential for use in existing composite structures or permanently integrated during the manufacturing process and is in situ and non-invasive.     

Model for thermal conductivities in spun yarn carbon fabric composites

A study on the thermal conductivities of spun yarn‐type carbon/phenolic (spun C/P) composites using a thermal‐electrical analogy method is presented. This method is based on the similarity between the partial differential equation that governs the thermal potential and electrical potential distribution. The unit cell of a laminate composite is divided into several conduction elements. By constructing an equivalent thermal resistance network in series, and in parallel based on analogy, we were able to predict the thermal conductivity of the composite. The prediction values obtained from the model are compared with known thermal conductivities on a carbon/epoxy composite with an eight‐harness satin (8HS) texture. It is shown that the model provides a good estimate of the thermal conductivity of the spun yarn fabric‐reinforced composite. With the use of this model, the thermal conductivity of the spun C/P composites with 8HS was validated experimentally. Good agreement was found between the present approach and the experimental results. POLYM. COMPOS., 26:791–798, 2005. © 2005 Society of Plastics Engineers     

Thermophysical properties of carbon/graphite fibers and MOD-3 fiber-reinforced graphite

The flash method is extended to measure directly the thermal diffusivity of graphite/carbon fiber in unidirectionally fiber-reinforced composites and also in fiber bundles. The thermal diffusivity was measured using both composities and fiber bundles for Morganite II and Thornal 50 S graphite fiber and for composite samples containing PX 505 carbon fiber. In addition, the thermal diffusivity of Morganite II and Thornel 50 S graphite fiber was calculated from the effective thermal conductivity of composite samples measured by an absolute method. The thermal diffusivity of MOD-3 fiber-reinforced graphite was measured and the results were used to compute the thermal conductivity in the three orthogonal directions.     

Effect of silane coupling agent on the dielectric and thermal properties of DGEBA-forsterite composites

Diglycidyl ether of bisphenol A (DGEBA) -forsterite composites have been prepared through mechanical mixing process and the influence of silane coupling agent on the microstructure, dielectric and thermal properties were studied. Phase pure forsterite (Mg₂SiO₄) powder was prepared through solid state ceramic route. Filling fraction of forsterite in DGEBA matrix was varied from 10 to 40 vol%. The morphology and filler distribution of filled composite were studied by Scanning Electron Microscopy. Waveguide cavity perturbation technique was employed to measure the dielectric properties of composites. It is found that aminosilane treatment increased the dielectric constant and dielectric loss of the composites in both microwave and radio frequency ranges compared to composites prepared using untreated powders. Coefficient of thermal expansion of composites decreased with the forsterite addition and attains a relatively low value of 45 ppm/°C for composite containing 40 vol% surface treated filler.     

Residual Stresses in Microcomposites and Macrocomposites

Existing models for built-in residual stresses in composite materials are reviewed and discussed. In particular, the thermal longitudinal stress present in the fiber prior to a single-fiber fragmentation experiment is studied using various model composite data. It is found that this stress is typically compressive in nature and that, quantitatively, it depends on the fiber content, the degree of undercooling, and the thermoelastic constants of the fiber and the matrix. In the case of single-fiber composites (or microcomposites), the thermal longitudinal stress present in the fiber is high enough to either induce fiber sinewave buckling (such as in E-glass/epoxy), or extensive fiber fragmentation (such as in graphite HM/polypropylene) that may then be used to measure the dependence of compressive fiber strength upon length. This has to be accounted for in quantitative models that calculate interfacial adhesion parameters using single-fiber tests, such as the fragmentation test or the microbond test. Implications for high fiber content composites (or macrocomposites) are discussed.     

Extension of the hole-drilling method to birefringent composites

A complete stress analysis and reliable failure criteria are essential for important structural applications of composites in order to fully utilize their unique properties. The inhomogeneity, anisotropy and inelasticity of many composites make the use of experimental methods indispensable. Among the experimental techniques, transmission photoelasticity has been extended to birefringent composites in recent years. The extension is not straight-forward, in view of the complex nature of the photoelastic response of such model materials. This paper very briefly reviews the important developments in the subject and then describes the theoretical basis for a new method of determining the individual values of principal stresses in composite models. The mehod consists in drilling very small holes at points where the state of stress has to be determined. Experiments are then described which verify the theoretical predictions. The limitations of the method are pointed out and it is concluded that valuable information concerning the state of stress in a composite model can be obtained through the suggested method.     

Influence of Matrix Properties on Fragmentation Test

Physical aging was used to vary the mechanical properties of model single fiber composites without changing the chemistry at the interface in order to study how property changes affect the measurement of interfacial adhesion by the fragmentation test. The properties of epoxy matrix/AS4 single fiber composites driven to full cure (T_g = 166°C) are altered by annealing below T_g. Neat resin samples with identical thermal histories are tested. All aged panels show roughly the same embrittlement with aging characterized by an average 30% decrease in tensile failure strain and 7.3% increase in compressive yield relative to quenched samples. Fragmentation results indicated no change between aged and quenched samples. Results are discussed in terms of micromechanics models for the fragmentation test. Strain at fragmentation increased with aging. This was related to the residual stress state in the model composite and the possibility of the zero stress state of the single fiber composites increasing with thermal annealing.     

连续纤维复合材料环形试样蠕变行为研究

对连续玻璃纤维复合材料进行了拉伸蠕变试验研究,为了模拟复合材料在压力容器中的受力状态并减少夹具加持力对试样的影响,采用环形复合材料试样拉伸蠕变试验方法。对复合材料环形试样的拉伸强度及不同应力等级下的拉伸蠕变性能进行了研究,并基于时间?应力等效原理,通过双对数法拟合出压力容器50年使用寿命时复合材料的最大蠕变应力,为复合材料压力容器的设计提供支持。并基于时间?应力等效原理,通过双对数法拟合出压力容器50年使用寿命时复合材料的最大蠕变应力应低于其拉伸强度的44.4 %。     

Exploration of Epoxy Resins,Hardening Systems,and Epoxy/Carbon Nanotube Composite Designed for High Performance Materials: A Review

Epoxy resins, is an important class of reactive polymers, have been reported to be toughened by nanoparticles. Carbon nanotube is a tubular cylinder ofcarbonatoms having extraordinary mechanical, electrical, and thermal properties. In this article, present state of epoxy/carbon nanotube composite is given. Types of epoxy and hardening agents commonly used in composite processing have been thrashed out. Frequently used fabrication techniques are discussed with particular emphasis on evaluating dispersion state of nanotube. Epoxy/carbon nanotube composites offer substantially improved properties compared to traditional fiber-reinforced epoxy composites. Finally, potential relevance for efficiently transforming filler properties to matrix facilitating aerospace relevance is conversed.